_rowaddr and _rowid not exposed for `merge_insert`

Sort of a follow up on #3251, I noticed that _rowid and _rowaddr doesn't seem to be usable for merge_insert, while it works for merge. When I try to use it with a subcol update, something like

import pyarrow as pa
import polars as pl

initial_data = pa.table(
    {
        "a": range(10),
        "b": range(10),
        "c": range(10, 20),
    }
)

dataset = lance.write_dataset(
    initial_data, "/tmp/lance/test2.lance"
)

new_values = pl.from_arrow(dataset.to_table(with_row_id=True)).select(pl.col("_rowid"), pl.col("a") * 2)

(dataset.merge_insert("a").when_matched_update_all().execute(new_values))

gives me

OSError: Append with different schema: fields did not match, missing=[b, c], unexpected=[_rowid], location: /Users/runner/work/lance/lance/rust/lance-core/src/datatypes/schema.rs:142:27

Start with the Python repro using dataset.merge_insert("a") and compare its handling of _rowid and _rowaddr with merge. The reported schema mismatch points to lance/rust/lance-core/src/datatypes/schema.rs:142;27. Done means the merge_insert operation can use the row identifiers without the reported missing and unexpected-field error.